1. Field of the Invention
The invention relates to the fabrication of electric lamps, and in particular to the fabrication of capsules for use in arc discharge lamps. Although the invention is applicable to arc discharge lamp tubes of any size, it is particularly useful to low wattage lamps.
2. Description of the Prior Art
Although relatively expensive to manufacture, metal halide arc discharge lamps are becoming increasingly popular due to their superior light output and long life compared to incandescent lamps. Metal halide lamps generally include an outer sealed envelope and an inner arc discharge capsule. The arc discharge capsule is electrically connected at one end of a pair of electrical connectors which are then sealed through the outer envelope. The exterior ends of electrical connectors are electrically connected to a lamp base attached to the exterior of the outer envelope.
Typically, the fabrication of an arc discharge capsule for a metal halide lamp includes a series of separate steps. For example, one known method includes the following steps: (1) Starting with a quartz tube five inches long, one end is heat sealed. (2) The tube, with the open end up, is then loaded in a vertical lathe chuck. (3) The tube is rotated in the lathe and heated near the sealed end to its working temperature. (4) The molten glass is captured in a two-part mold and nitrogen is blown into the tube through the open end. The positive pressure causes the heated glass tube to fill the mold to form a blown capsule. (5) The blown capsule is trimmed to length. (6) The capsule is acid-washed, dried and vacuum-baked to reduce possible contamination. (7) An electrode assembly is sealed into one end of the capsule. (8) An exhaust tube is sealed onto the body of the capsule. (9) A second electrode assembly is sealed into the opposite end of the capsule. (10) The capsule is again acid-washed, dried and vacuum-baked. (11) The capsule is then exhausted and filled with a metal halide lamp fill through the exhaust tube. (12) The lamp capsule is finally tipped off to seal in the lamp fill.
As is readily apparent from the number of individual steps involved, the cost of manufacturing such a lamp capsule is high. There are numerous chances for contaminants to be introduced into the process, hence the need for the washing, drying and baking steps during the course of fabrication.
Use of metal halide lamps in areas requiring high reliability, and miniaturization, such as the auto industry, has intensified the need to avoid the long and costly manufacture process. In addition, new problems have arisen. For example, accurate positioning of the electrodes is more critical, especially in anode placement for direct current lamp designs. Further, a more uniformly shaped volume and a tipless lamp capsule are preferred features to avoid degradation of the optical qualities. All of these features are particularly difficult to achieve in small arc discharge lamps, for example those having a volume less than half a milliliter.
The prior art has addressed some of these problems For example, U.S. Pat. No. 4,540,373 to Rothwell et al., which issued on Sep. 10, 1985, and is assigned to the same assignee as the invention deals primarily with maintaining precise electrode alignment during formation of the press seals.
In U.S. Pat. No. 3,419,947 to Gottschalk et al., which issued on Jan. 7, 1969, electrode alignment is effected by maintaining the two electrodes in engagement during assembly and sealing, and then softening the lamp to allow the electrodes to be disengaged and adjusted to the desired spacing.
In U.S. Pat. 3,939,538 to Hellman et al., which issued on Feb. 24, 1976, a press mold is provided for the ends of an arc lamp tube, which facilitates electrode placement and improves color uniformity, but at the cost of a complex fabrication process.
Several fabrication steps are combined in U.S. Pat. No. 3,685,880 to Sobieski, which issued on Aug. 22, 1972. In this patent both electrodes are assembled into the capsule simultaneously, although this requires precise alignment of the assembly apparatus.
In U.S. Pat. No. 4,389,201 to Hansler et al., which issued on Jun. 21, 1983, further attempts are made at reducing the handling of a lamp during fabrication, but again this reduction comes at the expense of complex equipment and subsequent increased costs.
Similar examples of methods of improving the manufacture of miniature lamps are offered in U.S. Pat. Nos. 4,434,386 and 4,756,701. The former, to Lowe, which issued on Feb. 24, 1984, and is assigned to the same assignee as this invention, combines molding and press sealing operations into a single step by maintaining both electrodes at the same end of the tube. The latter, to Danko et al., which issued on Jul. 12, 1988, describes a method of making an incandescent capsule which is quite similar to the standard procedure noted above, but the cleaning steps have been eliminated.
There is a need for a fabrication technique which reduces costs by simplifying the manufacturing process, and still address the specific problems encountered in low wattage lamps. There is also a need to provide a fabrication technique which provides a means of positioning electrodes in an capsule in a repeatable manner. It is also desirable to provide a lamp capsule having a more uniformly shaped volume and one which is tipless. It is also desirable to achieve these features in a small volume arc discharge lamp.